This invention relates to electrodes and more particularly to membrane reference electrodes for use in analyzing specific ion concentrations in batteries and other ionic solutions.
In the development of a high temperature nonaqueous energy cell, it is important that the concentration gradients of the cell be known. One manner of analyzing for concentration and other electrolyte properties is to generate a known reference electrode potential with an electrical double layer in the reference electrode, thereby measuring the potential difference between the reference electrode and the electrode of interest. Electrolytic solutions of some battery cells can have cell operating temperatures of between 100.degree. and 600.degree. C. At a cell operating temperature of about 400.degree. C., temperature variatons across a cell on the order of about 50.degree. C. have been detected; electrolyte concentrations also vary within a cell; such variations cause the EMF of the half cell to vary considerably within the confines of the cell. The change of voltage, if undetected, prevents an effective determination of cell voltages under the various conditions of cell operations. For example, temperature effects may cause the cell voltage to change by values in the order of at least 12 mV; it becomes important to detect changes in the cell voltage along the orders of 3 to 4 mV with an accuracy of about 1 mV. In addition, the concentration ingredients of the various components in the electrolytic solution can vary significantly throughout the cell volume, causing similar variations in EMF across the cell. It becomes very important in the analysis of the efficiency and operation of a battery cell to measure and calculate the potential of the electrolytic solution at various precise points within the cell in order to develop a working knowledge of the cell and cell efficiency.
Previous reference electrodes and other membrane sensors have been characterized by elongated tubes of ceramic or similar material with a spherical membrane having an area of one square centimeter or more forming the tip. In general the membrane material has been of low conductivity; the relatively large area of the membrane contacting the solution resulted in a low spatial resolution of potential and activity gradient in the electrolytic solution.